Process for Producing Ethanol

ABSTRACT

A process for producing ethanol including a combination of biochemical and synthetic conversions results in high yield ethanol production with concurrent production of high value coproducts. An acetic acid intermediate is produced from carbohydrates, such as corn, using enzymatic milling and fermentation steps, followed by conversion of the acetic acid into ethanol using esterification and hydrogenation reactions. Coproducts can include corn oil, and high protein animal feed containing the biomass produced in the fermentation.

FIELD OF THE INVENTION

This invention relates to a process for the conversion of carbohydratesfrom any of a number of sources into ethanol for fuel or chemical use.The invention uses a combination of fermentation and chemical conversionto greatly increase the yield of ethanol from carbohydrates compared tothe current art. In addition a high value coproduct may be produced foruse in animal feed.

BACKGROUND OF THE INVENTION

Ethanol is a major chemical used in human beverages and food, as anindustrial chemical, and as a fuel or a component in fuels, such asreformulated gasoline to reduce emissions from automobiles. Thisinvention relates mainly to the production of ethanol for use as achemical or fuel.

There are several traditional ethanol processes based on fermentation ofcarbohydrates. In the most typical process, a carbohydrate derived fromgrain is hydrolyzed to its component sugars and fermented by yeast toproduce ethanol. Carbon dioxide is generated in the process from afraction of the carbohydrate by the metabolism of the yeast. -Thegeneration of carbon dioxide is inherent in the metabolism of the yeast.This production of CO₂ by yeast limits the yield of ethanol from yeastto about 52% maximum on a weight basis. This is a major limitation onthe economic production of ethanol as the CO₂ is of low value and istypically wasted into the atmosphere and may become a burden on theenvironment.

In addition, yeast have a limited ability to utilize sugars other thanglucose. While glucose is the major sugar produced from the hydrolysisof the starch from grains, it is not the only sugar produced incarbohydrates generally. A large research effort has gone into thepotential conversion of biomass into ethanol. Biomass in the form ofwastes from agriculture such as corn stover, rice straw, manure, etc.,and biomass crops such as switch grass or poplar trees, and evenmunicipal wastes such as newspaper can all be converted into ethanol.However a major limitation of these processes is the complexity of thehydrolyzate that results from treatment of the biomass to produce thefermentation medium. The hydrolyzate typically contains glucose, butalso large amounts of other sugars such as xylose, which yeast cannotmetabolize. This is another potential yield limitation on yeast basedethanol processes.

Research has been directed to the use of organisms other than yeastwhich in. contrast to yeast, do consume many if not most of the sugarsderived from the hydrolysis of biomass. Examples include Zymomonas sp.bacteria and E. coli bacteria which have been genetically engineered toutilize xylose. Thereby the potential range of substrate. sugars whichcan be converted to ethanol has been increased. There is a class oforganism that has been proposed for the production of ethanol, typicallyof the Clostridium sp. These thermophiles usually produce both aceticacid and ethanol. However, it is believed that these organisms produce alimited yield of ethanol. It is generally assumed in the literature onethanol fermentation that this yield limitation is fixed by thebiochemical pathway called the Embden-Myerhof pathway by which ethanolis produced in all of the organisms so far proposed for production ofethanol, including the thermophiles.

Thus none of this development has addressed the inherent problem of theyield of ethanol from sugar based on the coproduction by the organismsof CO₂.

An important part of the commercial processes for producing ethanol isthe production of valuable coproducts mainly for use in animal feed orfood. In the corn dry milling process the coproducts include distillersdried grains and solubles (DDG, DDGS). In the corn wet milling processthe coproducts include germ, gluten meal and fiber. These coproductsfind large markets in the animal feed business. However in bothprocesses to a very large extent, the ingredients in the original grain,that is the oil, protein and fiber fractions, are passed through theprocesses unchanged in composition, while the carbohydrate fraction isconverted largely to ethanol. Therefore the value of these coproducts isbased on the inherent composition of the plant components.

There are other chemicals that can be produced by industrialfermentation from carbohydrates besides ethanol. Major examples areacetic acid and lactic acid. Acetic acid is a major food ingredient inthe form of vinegar and a major industrial chemical. Vinegar for fooduse is typically produced from potable ethanol by the action ofAcetobacter sp. which oxidize ethanol to acetic acid using oxygen fromthe air.

Major industrial uses for acetic acid are as a solvent, as anintermediate in the synthesis of other chemicals such as vinyl acetateand in the production of cellulose acetate. Major new uses for aceticacid have been proposed such as the production of calcium magnesiumacetate (CMA) for use as a road deicer in place of sodium chloride(NaCl). CMA has a much reduced environmental impact compared to NaClsince it is much less corrosive and is biodegradable.

Researchers have proposed the production of industrial grade acetic acidby fermentation from carbohydrates. However no production byfermentation currently exists due to economic factors related mainly torecovering acetic acid from dilute fermentation broths. Acetic acid istypically produced at low concentrations of around 5% or less in wateras a fermentation broth. Since acetic acid has a higher boiling pointthan water, all of the water, about 95% of the broth, must be distilledaway from the acetic acid to recover the acid or other more complexprocesses must be used to recover the acetic acid.

Related to this field of acetic acid production is the use of so calledacetogens, a class of bacteria which utilize a unique biochemicalpathway to produce acetic acid from sugars with 100% carbon yield. Forexample, one mole of glucose can be converted to three moles of aceticacid by Clostridium thermoaceticum. These bacteria internally convertCO₂ into. acetate. These bacteria are called homofermentativemicroorganisms or homoacetogens. They do not convert any of thecarbohydrate to CO₂ and only produce acetic acid. Examples ofhomoactogens are disclosed in Drake, H. L. (editor), AcetogenesisChapman & Hall, 1994, which is incorporated herein by reference in itsentirety. In addition these homofermentative organisms typically converta wide range of sugars into acetic acid, including glucose, xylose,fructose, lactose, and others. Thus they are particularly suited to thefermentation of complex hydrolyzates from biomass. However this line ofresearch has not overcome the economic limitations of the acetic acidfermentation process to make it competitive with the natural gas basedroute.

Therefore, industrial acetic acid is today made from coal, petroleum ornatural gas. The major process is the conversion of natural gas tomethanol and the subsequent carbonylation of the methanol using carbonmonoxide directly to acetic acid. U.S. Pat. No. 3,769,329 describes thisprocess.

Related to the natural gas route, it has been proposed to produceethanol from. acetic acid by way of synthesis of esters of acetic acidproduced in this process, or a related modification, and subsequenthydrogenation of the esters. U.S. Pat. Nos. 4,454,358 and 4,497,967disclose processes to produce acetic acid from synthesis gas, which isthen esterified and hydrogenated to produce ethanol, and areincorporated herein by reference in their entirety. The hydrogenation ofesters to produce alcohols is well known. None of these processes arebased on the conversion of carbohydrates to ethanol.

There is another class of well known fermentations that have theproperty of converting carbohydrates at 100% carbon yield, usinghomofermentative lactic bacteria. These bacteria convert one mole ofglucose for example into two moles of lactic acid. The relevance of thisis that lactic acid may also be used as the substrate for fermentationto acetic acid by homofermentative acetogens again with 100% carbonyield. Two moles of lactic acid are converted into three moles of aceticacid by Clostridium formicoaceticum for example. Prior to the presentinvention, no one has been known to have devise a process to produceethanol in high yield from carbohydrates, which is the main objective ofthis invention.

SUMMARY OF THE INVENTION

In accordance with one embodiment the present invention, carbohydratesare converted to ethanol with very high carbon yield by a combination offermentation and chemical conversion, thus overcoming the majorlimitation of known processes for the conversion of carbohydrates toethanol. The present invention combines several chemical and biochemicalsteps into a new process with many advantages. The basic process of thisinvention comprises three steps:

1. Converting a wide range of carbohydrates, with very high carbon yield(>90% potentially) using a homoacetic fermentation (or a combination ofhomolactic and subsequent homoacetic fermentations) into acetic acid,

2. Recovering, acidifying (if necessary), and converting the acetic acidto an ester (preferably, the ethyl ester using recycled ethanolproduct), and

3. Hydrogenating the ester, producing ethanol, and regenerating thealcohol moiety of the ester.

The net effect of this process is to convert carbohydrates in very highcarbon yield to ethanol. No CO₂ is produced from carbohydrates as abyproduct of this process.

Another benefit of the current invention is the production of a highervalue byproduct due to the conversion of the plant proteins intobacterial single cell protein. The conversion of the plant protein intosingle cell bacterial protein increases the concentration of theprotein, restructures the protein to have a more valuable compositionfor animal feed in terms of essential amino acids, for example, andpotentially provides other benefits, for example, in milk production.

The conversion of the fiber fraction, and the cellulose and xylanfractions of the grain contributes to the overall yield of ethanol.

While the production of single cell protein and the utilization of fiberare important additional benefits of the invention, the yield factoralone is a major improvement and can be practiced on its own inconjunction with the corn wet milling process, without the production ofsingle cell protein or the utilization of cellulose fiber.

Advantages of the invention over the current state of the art caninclude one or more of the following:

1. Very high yield of product from raw material with obvious economicbenefits compared to known ethanol processes,

2. No production of CO₂ from carbohydrate by the process with benefitsto the environment, i.e. the much more efficient conversion of renewableresources to ethanol,

3. Inherently wide substrate range for ethanol production, i.e. a widerange of potential biomass sources and their component sugars, and

4. High value byproducts, e.g., single cell protein; restructuring ofplant protein, produced with high efficiency.

In one embodiment of the present invention a method to produce ethanolwith a very high yield is provided. The method includes the steps offermenting a medium which contains a carbohydrate source into acetate,acetic acid or mixtures thereof. The acetate, acetic acid, or mixturesthereof are chemically converted to ethanol. Preferably, at least about60%, more preferably at least about 80% and more preferably at leastabout 90% of the carbon in the carbohydrate source is converted toethanol. Essentially none of the carbon in the carbohydrate source isconverted into carbon dioxide. However, if hydrogen is produced later inthe process by steam reforming, carbon dioxide will be produced at thatstage. Preferably, the fermentation medium comprises less than about 20%nitrogen and yields a biomass byproduct which is useful as an animalfeed, with preferably at least about 10% by weight biomass product. Thecarbohydrate source can include any appropriate source such as corn,wheat, biomass, wood, waste paper, manure, cheese whey, molasses, sugarbeets or sugar cane. If an agricultural product such as corn isemployed, the corn can be ground to produce corn and corn oil forrecovery. The carbohydrate source, e.g., corn, can be enzymaticallyhydrolyzed prior to fermentation. Preferably, the fermentation isconducted using a homofermentative microorganism. The fermentation canbe a homoacetic fermentation using an acetogen such as a microorganismof the genus Clostridium, e.g., microorganisms of the speciesClostridium thermoaceticum or Clostridium formicoaceticum.

In an embodiment of the present invention, the fermentation includesconverting the carbohydrate source into lactic acid, lactate or mixturesthereof by fermentation and subsequently converting the lactic acid,lactate or mixtures thereof into acetic acid, acetate or mixturesthereof by fermentation. The lactic acid fermentation can be ahomolactic fermentation accomplished using a microorganism of the genusLactobacillus. Alternatively, the carbohydrate source can be convertedinto lactic acid, lactate, acetic acid, acetate or mixtures thereof inan initial fermentation using a bifido bacterium. Typically, one mole ofglucose from the carbohydrate source is initially converted to about twomoles lactate and the lactate is converted to about three moles acetate.

Acetic acid which is formed in connection with the fermentation can bein the form of acetate depending on the pH of the fermentation medium.The acetate can be acidified to form acetic acid. For example, theacetate can be reacted with carbonic acid in and an amine to formcalcium carbonate and an amine complex of the acetate. The amine complexcan be recovered and thermally decomposed to regenerate the amine andform acetic acid. The calcium carbonate can be recovered for reuse. Theacetic acid can be esterified and hydrogenated to form an alcohol.Alternatively, the acetic acid may be directly hydrogenated to formethanol. The esterification is preferably accomplished by reactivedistillation.

In another embodiment of the present invention, the acetate can beacidified with carbon dioxide to produce acetic acid and calciumcarbonate and esterified to acetate ester for recovery. Preferably, theprocess takes place at low or nearly atmospheric pressure. Preferably,the calcium carbonate is recycled to a fermentation broth in order tomaintain a desired pH. Preferably, the ester is a volatile ester. Asused herein, the term “volatile ester” means that the ester is capableof recovery by distillation, and therefore the ester should be morevolatile than the water from which it is recovered. The alcohol employedin the esterification is preferably methanol, ethanol or mixturesthereof The ester is preferably recovered by distillation, such as byreactive distillation, and subsequently converted to ethanol.

The reactive distillation can be accomplished by acidifying, esterifyingand recovering the ester in a reaction column. A dilute solution ofacetate salt in water mixed with ethanol is introduced near the top of areaction section of the column. Carbon dioxide gas is introduced nearthe bottom of the reaction section of the column. The carbon dioxidereacts with acetate salt and ethanol in the reaction zone to formcalcium carbonate and ethyl acetate. Ethyl acetate can be concentrated,e.g., by vaporizing a mixture containing excess ethanol in water and anazeotrope comprising ethyl acetate, water and ethanol. The azeotrope canbe separated from the excess ethanol and water, e.g., by the addition ofwater, thereby causing a phase separation between an ethyl acetate-richportion and a water and ethanol-rich portion. The ethanol and water canbe returned to the reaction zone and the calcium carbonate can berecycled to a fermentation broth to control pH.

In one embodiment of the present invention, ethanol is produced from acarbohydrate source, with essentially none of the carbon and thecarbohydrate source converting to carbon dioxide.

In another embodiment of the present invention, ethanol is produced froma carbohydrate source wherein at least 60%, preferably 70%, morepreferably 80% and more preferably 90% and more preferably 95% of thecarbon in the carbohydrate source is converted to ethanol.

In accordance with another embodiment of the present invention, an esteris recovered from a dilute solution of a carboxylic acid salt. Thecarboxylic acid salt is acidified with carbon dioxide to produce thecorresponding carboxylic acid and calcium carbonate, and simultaneouslyesterified with an alcohol to form an ester. The ester is recovered.Preferably, the ester is a volatile ester and the alcohol is methanol,ethanol or mixtures thereof The ester can be recovered by distillation,such as by reactive distillation. The ester can be converted to ethanol.The acidification, esterification and recovery can take place in areaction column. Initially, a dilute solution of the carboxylic acidsalt in water mixed with alcohol is introduced near the top of areaction section of the column. Carbon dioxide gas is introduced nearthe bottom of the reaction section of the column. The carbon dioxide andcarboxylic acid salt and alcohol react to form calcium carbonate and avolatile ester of the carboxylic acid salt. The ester can beconcentrated by vaporizing a mixture containing excess alcohol and waterand an azeotrope made up of the ester, water and alcohol. The azeotropecan be separated from the excess alcohol and water, e.g., by theaddition of water, thereby causing a phase separation between anester-rich portion and a water and alcohol-rich portion. The excessalcohol and water can be returned to the reaction zone.

In accordance with an embodiment of the present invention, acarbohydrate source and natural gas are converted to an easilytransportable liquid product. The carbohydrate source is converted toacetic acid, acetate or mixtures thereof by fermentation. The aceticacid, acetate or mixtures thereof is converted to ethyl acetate. Atleast part of the ethyl acetate is converted to ethanol using hydrogenobtained from the natural gas source. The ethanol and/or ethyl acetatewhich is produced is then transported to a location remote from where itis produced. Preferably, the carbohydrate source and natural gas sourceare located within a distance that makes it economically feasible toproduce the transportable liquid product, and the remote location is asufficient distance away that it is not economically feasible totransport the carbohydrate and natural gas to the remote location forprocessing. Preferably, the economically feasible distance is less thanabout five hundred miles and the uneconomical remote distance is greaterthan about a thousand miles,. For example, the natural gas source andcarbohydrate source can be located on a Caribbean island such asTrinidad and the remote location can be on the Gulf Coast, such as theTexas Gulf Coast. Alternatively, the carbohydrate source and the naturalgas source can be located in Australia and/or New Zealand and the remotelocation can be Asia, e.g., Japan, Taiwan, Korea or China.

In another embodiment of the present invention at least 80% of thecarbon in a carbohydrate source is converted into ethanol. The methodincludes enzymatically hydrolyzing the carbohydrate source to sugars andamino acids. A carbohydrate, sugars and amino acids (from the originalsource or another source) are converted into lactic acid, lactate ormixtures thereof by homolactic fermentation. The lactic acid, lactate ormixtures thereof are converted into acetic acid, acetate or mixturesthereof by homoacetic fermentation. The pH of the fermentation brothsare maintained in a range from about pH 6 to about pH 8, using a base. Abiomass byproduct which is useful as an animal feed can be recoveredfrom the fermentation. The acetate is acidified with carbon dioxide toproduce acetic acid and calcium carbonate and the acetic acid issimultaneously esterified with an alcohol to form a volatile ester. Thevolatile ester can be recovered using reactive distillation. Hydrogencan be produced by any number of methods, e.g., steam reforming ofnatural gas. The acetate ester is hydrogenated to form ethanol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of the process of thepresent invention.

FIG. 2 illustrates the metabolic pathway for the conversion of glucoseto lactate.

FIG. 3 illustrates the metabolic pathway for the conversion of glucoseto acetate.

FIG. 4 illustrates one embodiment of reactive distillation.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the ethanol process of the present invention uses aunique combination of enzymatic milling, indirect fermentation, andchemical synthesis to produce a slate of high valued products. FIG. 1 isa simplified block flow diagram for the process. Corn 10 is ground 20the oil 30 is extracted, and the remaining material is thenenzymatically converted 40 using enzymes 44 into fermentable sugars andamino acids. Acetic acid and bacterial biomass 60 are produced by a twostep fermentation 50. The first step uses a lactic acid bacteria such asLactobacillus casei to convert the fermentable sugars into lactic acid.The second fermentation step uses an anaerobic bacteria such asClostridium formicoaceticum to convert the lactic acid and residualsugars from the first fermentation into acetic acid without any CO₂byproduct. This combination of fermentation steps results in a highyield of acetic acid with the coproduction of bacterial biomass 60 thatcan meet the US FDA requirements on direct-fed microbials. The resultingacetic acid is converted into ethanol 70 using chemical synthesis steps(esterification+hydrogenation) 80. The bacterial biomass 60 from thefermentations is directly usable or can be processed into a high proteinanimal feed concentrate called

Single Cell Protein (SCP) 60.

The overall chemistry for the major steps are as follows:

Enzymatic Treatment: (1/n) Starch + H₂O → Dextrose Fermentation 1:Dextrose → Lactic Acid Fermentation 2: 2 Lactic Acid → 3 Acetic AcidEsterification: 3 Acetic Acid + 3 Ethanol → 3 Ethyl Acetate + 3 H₂OSteam Reforming: 1.5 Methane + 3 H₂O → 1.5 CO₂ + 6 H₂ Hydrogenation: 3Ethyl Acetate + 6 H₂ → 6 Ethanol Overall: (1/n) Starch + 1.5 Methane +H₂O → 3 Ethanol + 1.5 CO₂where n is the number of dextrose units in a starch molecule. The aboveequations show starch as the only source of fermentable sugars in corn.However, the ethanol process of the present invention uses an enzymaticmilling process which also makes the cellulose and hemicellulosefractions of corn available for fermentation. The enzymatic millingprocess increases the amount of fermentable sugars derived from a givenamount of corn by about 20% over traditional wet milling.

Another reason for the high yield of the ethanol process of the presentinvention is the amount and source of the CO₂ produced by the presentprocess. In the ethanol process of the present invention, only 0.5 molesof CO₂ are produced for every mole of ethanol. In contrast, traditionalfermentation routes produce 1 mole of CO₂ for every mole of ethanol.Furthermore, the ethanol process of the present invention uses lessexpensive methane rather than dextrose as the carbon source for carbondioxide. Lower CO₂ production from less expensive feedstocks leads tobetter process economics.

Preparation of Suitable Fermentation Substrate

There are many processes which are well known in the state of the art toprovide suitable fermentation media for lactic acid or acetic acidfermentations. These can be media which minimize the amount of nitrogenin the media and thus minimize the amount of single cell protein. On theother hand there are processes which attempt to increase the utilizationof nitrogen in the feed.

Any suitable media preparation process may be used for the purposes ofthis invention.

As an illustrative example only, one can consider using corn as the rawmaterial. Several pretreatment steps are typically used in corn millingsuch as cleaning, germ removal for oil production, etc as is well knowto those in the milling art.

Typically enzymatic treatment is used to convert the corn into a mediathat is suitable for metabolism by the bacteria in the downstreamfermentations, although acid hydrolysis has also been used. The groundcorn is mixed with water to form a slurry which is then heat sterilized.A continuous sterilizer that heats the corn slurry to 120-140C. andprovides 1 to 5 minutes of residence time can be used. Preferably, theretention tubes are designed to provide turbulent flow (Re22 4000) withminimal dead spots, so that good sterilization without excessivecarmelization occurs.

Heat sterilization also begins the liquefaction process. Duringliquefaction the starch granules swell and the viscosity of the slurryrises dramatically. Heat stable a-amylase is used to limit the rise inviscosity by depolymerizing the starch molecules—a process calledsaccharification. a-amylase is an enzyme which hydrolyzes the 1,4linkages in the starch molecule. It is classified as an endoenzyme sinceit randomly attacks bonds in the interior of the starch molecule.Sufficient reduction in viscosity is achieved with 10-15% hydrolysis ofthe starch in less than 10 minutes residence time at pH 5.5-7.0.

Glucoamylase is preferably used to complete the hydrolysis of the starchmolecule. Glucoamylase is an exoenzyme since it only attacks the ends ofthe starch molecule. The enzyme hydrolyzes both 1,4 and 1,6 linkages, sonearly complete hydrolysis of the starch can be achieved. Optimalconditions for glucoamylase are typically 58-62C, pH 4.4-5.0, and 24-48hours of residence time. Longer residence times are typically notbeneficial since the enzyme also catalyzes the formation ofnon-fermentable disaccharides processes called reversion andretrogradation.

In addition to the utilization of the starch fraction of corn it isdesirable to utilize the other major fractions, including thehemicellulose and cellulose, as well as the protein in this invention.This is not typically done in current ethanol processes. The higheryield of this invention and the wide substrate utilization capability ofthe fermentation used enhance the value of these added steps as opposedto the current processes as will be shown.

Hydrolysis of hemicellulose can be carried out in several ways. Muchresearch is known on acid hydrolysis, but enzymatic hydrolysis is alsowell known. Complete enzymatic hydrolysis of hemicellulose requires amixture of enzymes. The pendant arabinose and glucuronic acids areremoved from the xylose backbone using a-L-arabinofuranosidase anda-glucuronidase. The xylose backbone is hydrolyzed usingendo-b-1,4-xylanase and b-xylosidase.

Cellulose utilization is also of value. Several methods are know for thehydrolysis of cellulose to fermentable sugars. For example, cellulose ishydrolyzed by the synergistic action of three cellulase enzymes:endo-b-glucanase, exo-b-glucanase, and b-glucosidase. Theendo-b-glucanase is an endoenzyme which randomly hydrolyzes the 1,4linkages in the interior of the cellulose molecule. Exo-b-glucanaseremoves cellobiose units (a disaccharide of b linked glucose) from thenon-reducing end of the cellulose chain. b-glucosidase hydrolyzes acellobiose unit into two glucose molecules. Working together, the threeenzymes can convert cellulose into glucose monomer. It is also a featureof this invention that lactic acid bacteria as used in this inventionutilize cellobiose directly which reduces feedback inhibition of thehydrolysis.

The hemicellulose and cellulose enzymes have been the focus of muchresearch work over the past 10-20 years. These enzymes are required forefficient conversion of woody biomass materials into fermentable sugars,which can then be used as fermentation feedstocks for ethanol and otherfermentation products by traditional processes. Biomass materials suchas grass, wood, paper, and crop residues are much less expensive thanstarch based materials such as corn starch.

Reduction in enzyme cost can be obtained by overlapping thesaccharification activity with the fermentation process in a designcalled Simultaneous Saccharification and Fermentation (SSF). Productinhibition of the cellulases is avoided by conversion of the glucoseinto ethanol or other desired fermentation product. The SSF philosophyhas been used for decades by the ethanol industry with starch enzymes.Research also shows that this concept works for the hemicellulase andcellulase enzyme systems. This process may also be used in the currentinvention. It is a preferred process because the fermentation used inthis invention utilizes more of the types of sugars produced in thehydrolysis and further accelerates the hydrolysis compared to a yeastfermentation which consumes the glucose fraction largely.

In addition to the utilization of the fiber fraction comprisinghemicellulose and cellulose, it may be desirable in this invention toutilize the protein fraction.

Protease enzymes are used to hydrolyze the corn proteins into smallerpeptides and amino acids. These amino acids and peptides are a majornitrogen source for the fermentation bacteria. Hydrolysis of theproteins is required to speed nitrogen assimilation in the fermentation.U.S. Pat. No. 4,771,001 shows the use of protease enzymes to increasethe utilization of proteins by a lactic acid fermentation. This patentalso illustrates the use of a different raw material, in this casecheese whey. For the purposes of the current invention the protein usedto supplement the fermentation can come from the corn as illustrated, orfrom other protein sources and can be mixed into the media. Any proteinsource that produces a suitable fermentation media for lactic acid oracetic acid fermentation and does not inhibit the fermentation may beused.

In its most general embodiment the current invention does not dependupon a specific carbohydrate or protein source, but any suitable sourcemay be used.

Fermentation The overall purpose of the fermentation part of the currentinvention is to convert the fermentable carbohydrates and amino acidsinto acetic acid and single cell bacterial protein. In a preferredembodiment a two step fermentation process is used. The first step usesa homofermentative lactic acid bacteria to convert the bulk of thefermentable sugars into lactic acid and single cell protein. The secondstep uses a homofermentative acetogenic bacteria to convert lactic acidand residual carbohydrates into acetic acid.

The lactic acid fermentation step uses a homofermentative lactic acidbacteria such as Lactobacillus casei to convert the fermentable sugarsinto lactic acid. Lactic acid bacteria are gram-positive, non-sporeforming, aerotolerant anaerobes. These bacterial are found in the mouthsand intestinal tracts of most warm blooded animals including humans.None are pathogenic and many are approved by the US FDA as viableorganisms for use in direct-fed microbials for animal feeds. Viablecultures are also present in many yogurts consumed by humans.

As shown in FIG. 2, lactic acid is the sole metabolic product forhomofermentative strains. Glucose is metabolized to pyruvate using theregular Embden-Meyerhof glycolytic pathway. Pyruvate is convert tolactic acid in a single NAD coupled step. Most lactic acid bacteria aremesophilic with optimal temperatures for growth between 35 to 45C. Cellgrowth is pH sensitive with optimal pH around 6.0. Product inhibitionbegins to affect the kinetics of cell growth and acid production atlactic acid levels above 4 wt %. Complete inhibition of growth occursaround 7 wt % while complete inhibition of acid production occurs around10-12 wt %.

The feed to the fermentation is very dilute in carbohydrates with onlyabout 5 wt % fermentable sugars. A single stage continuous stirred tankreactor (CSTR) type fermentor is appropriate for this step. However, anysuitable bioreactor can be used, including batch, fed-batch, cellrecycle and multi-step CSTR. The low carbohydrate concentration in thefeed will limit the effects of product inhibition on the cell growth andacid production kinetics, thus 90+% conversion of the dextrose withabout 18-24 hour residence times is possible. Most homofermentativestrains will readily metabolize a range of substrate sugars. It isadvantageous to combine the lactic acid fermentation with the subsequentacetic acid fermentation in such a manner so as to utilize all of thesugars.

In contrast to many industrial lactic acid fermentations, the currentinvention may be operated in a mode in which the fermentation iscarbohydrate limited rather than nitrogen limited. Thus biomassproduction is maximized by keeping most of the fermentation in thegrowth associated state and ensuring that sufficient nitrogen isavailable for growth. For any growth associated fermentation the biomassyields are typically about 10.5 g per mole of ATP produced. Since lacticacid fermentations produce a net of 2 moles of ATP per mole of glucose,the biomass yield will be around 2 (10.5/180)=0.12 g per g of glucose.By stoichiometry, the remaining 0.88 g of glucose are converted into0.88 grams of lactic acid.

The efficient production of biomass as single cell protein is animportant part of this invention. In contrast to the production ofsingle cell protein historically, the use of an anaerobichomofermentative fermentation is very advantageous. This is because allof the energy production of the organism comes from the production ofthe desired metabolite whether lactic acid or acetic acid. This meansthat there is no wasted byproduct CO₂ as is the case in aerobicfermentations. In addition, because of the lack of production of CO₂,the heat produced by the fermentation is also minimized. Therefore theutilization of energy contained in the raw material carbohydrates ismaximized toward the production of valuable single cell protein orlactic and acetic acid. The traditional yeast fermentation, in additionto wasting mass as CO_(2,) also requires the removal of heat.

The fermented broth from the first fermentation step is clarified usinga centrifuge. The concentrate contains the lactic acid bacteria and issent to single cell protein recovery. The amount of single cell proteinproduced is related to the amount of nitrogen in the form of hydrolyzedproteins as amino acid and peptides that is supplied to the fermentationin the medium. This can range from a very small amount, but not zero, aslactic acid bacteria require some complex nitrogen sources, such as 1%up to about 15% overall yield of single cell protein based on the totalnitrogen plus carbohydrate in the medium. It is a feature of theinvention that the production of single cell protein can be controlledover a wide range. The single cell protein can be processed by anysuitable means, such as spray drying, to produce a salable product.

Another important feature of the current invention is the production ofa single cell protein which is enhanced in value as an animal feedingredient. The single cell protein from the lactic acid fermentationhas these features. It has a high protein concentration of about 70%,depending on the strain of organism and the specific conditions of thefermentation. It has a good amino acid profile. That is, it contains ahigh percentage of so called essential amino acids comprising, forexample, lysine, methionine, isoleucine, tryptophan, and threonine. Thecombined percentage of these amino acids in lactic acid bacteria isabout 10.5%, compared to corn protein which has about 1% of the totalcorn kernel. The protein composition of corn depends on the fraction ofthe corn considered. Corn gluten meal, for example, has about 7.5%, butcorn gluten feed has about 2.5% of essential amino acids. This enhancedamino acid composition is directly related to the value of the proteinas an animal feed ingredient.

In a preferred embodiment, the current invention can produce single cellprotein at high efficiency and with high value.

The centrate, from the separation of the lactic acid bacteria from thefermentation broth of the first fermentation, is fed to a secondfermentor where the lactate is converted into acetate using anacetogenic bacteria. Lactate can be a preferred substrate for acetogenicbacteria in many of their natural environments. The rate of fermentationand yield on lactate substrate can be very high, e.g., over 98% yield ofacetate from lactate.

Incomplete removal of the lactic acid bacteria is typically acceptablesince the acetic acid fermentation typically uses a thermophilic strainand the second fermentation is done at a higher temperature.Contamination of the acetic acid fermentation with a mesophilic lacticacid bacteria is typically not an issue since the lactic acid bacteriatypically cannot grow at these higher temperatures. Also, near completeconversion of the glucose is expected in the first fermentor, so thelactic acid bacteria which do happen to bleed through the centrifugeinto the second fermentor will not have a carbohydrate source.

The acetogenic bacteria have been known and studied since the 1930′s.Drake, H. L. (editor), Acetogenesis, Chapman & Hall, 1994, gives a goodoverview of the field. The acetogenic bacteria include members in theClostridium, Acetobacterium, Peptostreptococcus and other lesser knownspecies. The habitats of these bacteria are: sewers, anaerobic digestersat municipal waste treatment plants, natural sediments, termite guts,rumens, and intestinal tracts of non-ruminants including humaris.Pathogenicity is rare. All of these organism are strict anaerobes, whichmeans that contact with oxygen is often fatal to the microorganism.Clostridium are spore formers. Spores are resistant to manysterilization techniques and special procedures have been establishedfor handling spore-forming bacteria. The Acetobacterium andPeptostreptococcus species are not spore formers.

FIG. 3 is a simplified sketch of the metabolic pathways used by mostacetogenic bacteria. The organism metabolizes glucose to pyruvate usingthe normal Embden-Meyerhof glycolytic pathway. Lactic acid is alsometabolized by first converting it back to pyruvate. From pyruvate, theorganism makes acetic acid and carbon dioxide using the regularoxidation pathways. The main distinguishing feature of acetogenicbacteria is that the CO₂ produced in this oxidation step is not releasedto the environment. Instead, the acetogenic bacteria have a metabolicpathway which will fix the CO₂ and make an additional mole of aceticacid.

The novel acetogenic pathway provides three functions for the organism:

1. Like all anaerobes, a terminal electron acceptor other than oxygen isrequired to balance the redox reactions of metabolism. In this case, thereduction of carbon dioxide acts as the electron sink.

2. Cellular energy (i.e. ATP) is produced from this pathway. Themetabolic pathways for conversion of one mole of glucose into two molesof acetic acid and two moles of carbon dioxide produce four ATP per moleof glucose consumed. Addition of the acetogenic pathways creates anotheracetic acid molecule from the carbon dioxide and increases the ATP yieldto 4-6 ATP per mole of glucose. The additional ATP are not made directlyfrom the substrate-level phosporylation but are made in other processessuch as the electron transport chain and from ion pumps located in thecell membranes. The exact amount of ATP produced from the secondarysources varies from strain to strain and is also dependent upon the cellenvironment.

3. Carbon dioxide can be converted into cellular carbon needed forgrowth using the cell's anabolic pathways, even when common carbonsources such as glucose are not available.

Some acetogens will produce other organic acids such as formic,propionic, succinic, etc. in addition to acetic acid. These organismsare described as heterofermentative as opposed to the homofermentativeorganisms which only produce acetic acid. The heterofermentativepathways represent a potential yield loss in the current invention, andproper strain selection and elucidation of the factors which cause theformation of these other organic acids will minimize the impact.

By far, most work to date has been with the Clostridium strains. Many ofthese strains are thermophilic with optimal temperatures for growtharound 60C. Several kinetic studies (Yang, S .T., Tang, I. C., Okos, M.R., “Kinetics and Mathematical Modeling of Homoacetic Fermentation oflactate By Clostridium formicoaceticum”, Biotechnology andBioengineering, vol. 32, p. 797-802, 1988, Wang, D. I., Fleishchaker, R.J.; Wang, G. Y., “A Novel Route to the Production of Acetic Acid ByFermentation”, AIChE Symposium Series-Biochemical Engineering: RenewableSources, No. 181, vol. 74, p. 105-110, 1978; and Tang, I. C., Yang, S.T., Okos, M. R., “Acetic Acid Production from Whey Lactose by theCo-culture of Streptococcus lactis and Clostridium formicoaceticum”,Applied Microbiology and Biotechnology, vol. 28, p. 138-143, 1988, whichare incorporated herein by reference in their entirety) have beenconducted to examine the effects of pH and acetate levels on both cellgrowth and acid production. These organism are sensitive to low pH andproduct inhibition occurs at much lower concentrations than in lacticacid bacteria. Optimal pH is around 7 and maximum acetate tolerance isonly about 30 g/l in batch fermentation.

A one or two stage CSTR fermentor design is typically appropriate forthe second fermentation step. However, any suitable bioreactor can beused, including batch, fed-batch, cell recycle, and multi-step CSTR. Incontrast to the first fermentation step, the acetic acid fermentation isnitrogen limited rather than carbohydrate limited. Yield of acetic acidfrom lactic acid can be greater than 85% of theoretical.

In one embodiment, the broth from the second fermentation step isprepared for the second part of the current invention which is thechemical conversion. As an example, the broth is clarified with acombination of a centrifuge and a microfilter. The centrifuge removesthe bulk of the biomass and reduces the size of the downstreammicrofilter by reducing its load. The microfilter permeate is sent to ananofiltration unit. The microfilter acts as a prefilter for thenanofiltration unit. The nanofiltration unit removes proteins,unconverted sugars, etc. which have molecular weights above about 300.The nanofiltration unit removes the bulk of the impurities in theacetate broth and produces a water white permeate that can be sent todownstream processes.

The concentrates from the centrifuge, microfilter and nanofilter may beprocessed to recover values useful in the single cell protein orrecycled to one of the fermentation steps. Alternatively, they may bedisposed of in any acceptable manner such as composting or incineration.

Although a preferred embodiment of the current invention utilizes twofermentation steps and the production of single cell protein, this isnot required in the most general case. A suitable medium for the aceticacid fermentation alone may be provided. Although single cell proteinmay not be produced, the increased yield form the carbohydrate sourcewill still provide an important advantage for the current invention.

In addition, it is not necessary to utilize the hemicellulose orcellulose fraction of the raw material in order to get the advantages ofthe current invention. An example is the utilization of the invention inconjunction with a corn wet mill where the medium would be almost purestarch and corn steep water produced by the mill. The current inventionwould still increase the ethanol yield compared to current technology by75% providing a huge economic advantage.

The key feature of the fermentation step is therefore the conversion ofcarbohydrate from any source into acetic acid.

Acidification and Esterification

In the next step of the invention, the acetic acid or acetate producedin the fermentation is converted to an ester of acetic acid, preferablymethyl or ethyl ester and more preferably ethyl ester. Any suitableprocess that will convert the acetic acid or acetate salt to the esteris acceptable as part of this invention.

Acetic acid is a weak organic acid with pKa=4.76. If the fermentation isconducted at near neutral pH (i.e. pH=7.0), the product of thefermentation will actually largely be an acetate salt rather than theacid. In the fermentation, any suitable base can be used to neutralizethe fermentation. The preferred neutralizing agent is Ca(OH)₂, which canbe supplied by CaO (lime) or calcium carbonate (CaCO₃) which can berecycled from later in the process. Other neutralizing agents can beused, such as NaOH or NH₄OH, as determined by the conditions required bythe fermentation organism. However, even the acetate salt is inhibitoryand the maximum concentration of acetate is usually limited to about 5%in the fermentation broth.

Thus, there are two problems in the recovery of acetic acid salts from asolution such as a fermentation broth. The acetate salt must usually beconverted to the acid, and the acid must be removed from the dilutesolution in water. In addition it is desirable to recycle the base usedto neutralize the fermentation to reduce costs and avoid potentialenvironmental impact.

The most typical route is the sequential acidification of the salt toproduce acetic acid and then the subsequent recovery of the acid. Evenafter the salt is converted to a dilute acid solution, there is stillthe need to recover the product from the water. Many different processapproaches have been proposed to recover such dilute solutions. Sinceacetic acid has a higher boiling point than water, the bulk of thewater, about 95% of the broth, must be distilled away from the aceticacid to recover the acid if simple distillation is used. Alternatively,some more complex process may be used to recover the acetic acid,usually in conjunction with solvent extraction. However this line ofresearch, that is, acidification with subsequent recovery from thedilute solution, has not overcome the economic limitations of the aceticacid fermentation process to make it competitive with the synthesis gasbased route. Therefore, all industrial acetic acid is currently madefrom synthesis gas derived from coal, petroleum or natural gas.

A number of methods have been proposed to acidify the acetic acid saltsolution. One method is the reaction of the acetate salt with a strongacid such as sulfuric acid to form acetic acid (HAc) and calcium sulfate(CaSO₄). The CaSO₄ precipitates and is easily separated from the aceticacid solution. However, this method requires the consumption of acid andbase and produces a byproduct waste salt that may become anenvironmental burden. Another method is bipolar electrodialysis thatsplits the salt into an acid and base (this does not work well with Casalts, but one could substitute Na in this case). Other routes toproduce dilute acetic acid from the salt are well known.

Reaction of a carboxylic acid salt with an amine and CO₂ with theprecipitation of CaCO₃ and the formation of an acid amine complex thatcan be extracted and thermally regenerated has also been proposed, asshown by U.S. Pat. No. 4,405,717, which is incorporated herein byreference in its entirety.

U.S. Pat. No. 4,282,323, which is incorporated herein by reference inits entirety, discloses a process to acidify acetate salts using CO₂ ina number of ways. In the referenced patent the acetic acid formed isremoved by a solvent to a separate phase.

Esterification of acetic acid to form ethyl acetate is a well understoodreaction:

Esterification is typically performed in the liquid phase. Theequilibrium constant for this reaction is 4.0 and is nearly independentof temperature. Acid catalysts for the reaction include: strong Bronstedacids such as sulfuric acid and methane sulfonic acid, acidic ionexchange resins, zeolites, and a number of other materials, includingcarbonic acid formed by the dissolution of CO₂ in water. The reactionrate is influenced by the type and concentration of catalyst, thereaction temperature, and the degree of departure from equilibrium.

Alternative routes exist that attempt to avoid the separateacidification and esterification steps. A carboxylic acid salt may bereacted directly with an alcohol such as ethanol to produce the esterdirectly. An intermediate step may be inserted to convert the Ca salt toan ammonia salt. In this step the dilute Ca(Ac)₂ is reacted with NH₃ andCO₂ to form NH₄Ac and CaCO₃ which precipitates. The ammonia salt ofacetic acid may then be esterified directly as shown by U.S. Pat. No.2,565,487, which is incorporated herein by reference in its entirety.

Preferred Approach

The preferred approach is to combine chemical and phase changeoperations into a new efficient process to directly produce a volatileester of acetic acid and distill the ester away from the broth.

The three parts are:

1) Acidification of the fermentation broth with CO₂ at low or nearlyatmospheric pressure to produce acetic acid and precipitate CaCO₃ whichcan be recycled directly to the fermentation as the base;

2) Simultaneous esterification of the formed acetic acid with analcohol, such as methyl or ethyl alcohol, to form a volatile ester, and

3) Reactive distillation to push the acidification and esterificationequilibria to high conversion.

Since esterification is an equilibrium reaction, high conversion can beobtained by driving the reaction to the right with continuous removal ofone or more products. Reactive distillation similar to that developed byChronopol for lactide synthesis (See U.S. Pat. No. 5,750,732, which isincorporated herein by reference in its entirety) and by EastmanChemical for methyl acetate production (see U.S. Pat. Nos. 4,939,294 and4,435,595 and Agreda, V. H., Partin, L. R., Heise, W. H., “High-PurityMethyl Acetate Via Reactive Distillation”, Chemical EngineeringProgress, p. 40-46, February 1990, which are incorporated herein byreference in their entirety) is an economically attractive method. U.S.Pat. No. 5,599,976, which is incorporated herein by reference in itsentirety, discloses the conversion of very dilute acetic acid to theester in a continuous reactive distillation process. Xu and Chaung (Xu,Z. P, Chuang, K. T., “Kinetics of Acetic Acid Esterification over IonExchange Catalysts”, Can. J. Chem. Eng., pp. 493-500,Vol. 74, 1996) showthat reactive distillation to produce the ester of acetic acid fromdilute solution is the preferred method to remove acetic acid from verydilute solutions, as are produced in the current invention. In thisconcept, the acetic acid flows in a counter current fashion to theesterifying ethanol in a distillation column. In the current invention,ethyl acetate is more volatile than acetic acid so the ethyl acetate isdistilled away from the liquid mixture and the esterification reactionis pushed to the right, thus enabling high conversions in a singlevessel. The process proposed here goes beyond these examples in that itscombines simultaneous acidification with the reactive distillationesterification. AU of the cited processes start with acetic acid (orlactic acid in the Chronopol case) and not a salt.

The net effect of the reactive distillation process, the preferredroute, is to remove the acetic acid from the dilute solution withoutvaporizing the water which forms the bulk of the stream.

In addition, the use of CO₂ as the preferred acidifying agent with theprecipitation of CaCO₃ allows the recycle of the neutralizing agent tothe fermentation without the consumption of chemicals. The CaCO₃ can beused directly in the fermentation or can be converted first to CaO bycalcination. The reactive distillation process 80 a is shown in FIG. 4.

Reaction section: The raw material, a dilute (5%) solution of calciumacetate 410 (Ca(Ac)₂) in water 414 is mixed with ethanol 418 and fed tothe column 422 at the top of the reaction section 424. CO₂ 420 is fed tothe column 422 at the bottom of the reaction section 424. Thesimultaneous reaction of CO₂ 420 with Ca(Ac)₂ 410 and ethanol 418 takesplace in the reaction zone 424 in the center section of the column 422with the formation of CaCO₃ 428 and ethyl acetate (EtAc) 432.

CO₂ (g)+H₂O→H₂CO₃

Ca(Ac)₂+H₂CO₃→CaCO₃ (s)+2HAc

2HAc+2EtOH→2EtAc

The most volatile component in the reaction mixture is the ethylacetate/water/ethanol azeotrope 436. The azeotrope composition is 82.6%ethyl acetate, 9% water and 8.4% ethanol and has a normal boiling pointof 70.2° C. The azeotrope 436 is removed from the reaction mixture byvaporization along with some EtOH and water. The bottom product from thereaction zone is a water and ethanol solution containing the suspendedCaCO₃ flowing to the stripping section.

Separation Section: In the upper separation zone 450 the azeotrope isseparated from the ethanol and water also vaporized from the reactionmixture. The ethanol water mixture 454 is recycled to the reaction zone424 and the overhead product is the azeotrope 436. The CO₂ is separatedfrom the overhead condensate and recycled to the column with makeup CO₂.The azeotrope can be broken by the addition of water, which causes aphase separation, with the water and ethanol rich phase returned to the.appropriate point in the reactive distillation column (not shown).

Stripping Section: Since excess ethanol is used to favor the forwardesterification reaction in the reaction section, the stripping section458 returns the excess ethanol to the reaction zone. In the strippingsection 458 the ethanol is removed from the CaCO₃-containing waterstream which is discharged from the column 422 and separated by a simpleliquid /solid separation 462, such as centrifugation or filtration, intothe solid base 466 for recycle and water 470.

The net effect of the reactive distillation process is to recover theacetic acid from the dilute salt solution thereby producing a relativelyconcentrated product stream at the top and without vaporizing the waterthat forms the bulk of the stream. The integration of the three sectionsreduces the energy requirement. The simultaneous removal of the productester shifts the esterification equilibrium and leads to higherconversion in a short time.

It is unusual to handle precipitates in a distillation system. However,in this case the precipitation reaction occurs in the bulk phase and isnot due to the concentration of the solution at a heat transfer surface,a common type of fouling. Ethanol beer stills in the corn dry millinghanol industry typically handle solids loading in the stripping sectionthrough the use of trays with simple construction and large openings.Alternatively, it would be possible to operate the reaction section inother configurations, such as a series of stirred tanks with a commonvapor manifold, to simulate the column reaction section.

The successful development of a low cost, low energy, integratedacidification, esterification and purification process for ethyl acetatewould potentially allow the economic production on an industrial scaleof major chemicals from renewable resources, which are now produced fromnon-renewable resources.

One major benefit of using renewable resources is the reduction of CO₂production with the replacement of fossil raw materials. There would bea benefit to the U. S. economy from the replacement of importedpetroleum with domestic renewable resources. The use of agriculturalcommodities to produce chemicals and liquid fuels without subsidy hasimportant benefits to the farm community in terms of product demand andstable markets and reduces the cost of U.S. government subsidies.

Hydrogenation

The third major step in the invention is the conversion of the ester ofacetic acid into two alcohols by hydrogenation. The hydrogenation ofesters to produce alcohols is a well-known reaction.

U.S. Pat. Nos. 2,782,243, 4,113,662, 4,454,358, and 4,497,967, which areincorporated herein by reference in their entirety, disclose processesfor the hydrogenation of esters of acetic acid to ethanol.

For the particular case at hand, hydrogenation can be performed ineither the liquid phase or the gas phase. Any suitable hydrogenationprocess can be used. This reaction is also an equilibrium reaction. Thereaction can be driven to the right by using high partial pressures ofhydrogen. Typical reaction conditions are 150-250C. and 500-3000 psidepending upon the desired conversion and selectivity. The reaction canbe catalyzed by any suitable hydrogenation catalysts, such as copperchromite, nickel, Raney nickel, ruthenium, and platinum. A copperchromite, nickel, or Raney nickel catalyst is preferred for thehydrogenation since these catalysts are not poisoned by water. In theliquid phase process, an alcohol such as ethanol is a good solvent.

In the gas phase process, the ethyl acetate feed is vaporized and fed tothe hydrogenation reactor with an excess of hydrogen. After passingthrough the bed, the vapors are cooled and flashed into a low pressureknockout drum. The hydrogen rich vapor phase is recycled back to thereactor. The liquid phase is distilled to remove residual water andunreacted ethyl acetate. The water is not made by the hydrogenationchemistry; it's source is the liquid-liquid equilibrium level present inthe upstream reflux drum of the reactive distillation column.

Another distillation column may be needed as a final polishing step,depending upon the nature and quantities of side products from theesterification and hydrogenation units.

The preferred ester is ethyl acetate, as it avoids the introduction of asecond compound into the process which must be purified away from theproduct stream.

The water stripper collects water streams from the acidification,esterification, and hydrogenation units. The water is steam stripped torecover solvent values, then the water is sent to final treatment anddischarge or recycled to the fermentation section.

Many potential sources of hydrogen for use in the present inventionexist. Any suitable hydrogen source can be used that produces hydrogenof sufficient purity for the hydrogenation reaction and that will notpoison the catalyst. Raw materials for hydrogen production include waterfrom which hydrogen can be produced by electrolysis. Many fossil andrenewable organic feedstocks can also be used. If a fossil feedstock isused, such as methane from natural gas, some CO₂ will be produced alongwith the hydrogen. However, if a renewable feedstock is used then theCO₂ production will be neutral to the environment. For example,feedstocks which contain carbon and hydrogen at the molecular level canbe used to produce hydrogen. Wood chips, sawdust, municipal wastes,recycled paper, wastes from the pulp and paper industry, solidagricultural wastes from animal and/or crop production are all examplesof renewable feedstocks that can be used for hydrogen production, e.g.,using gasification technology.

Steam reforming of methane to produce hydrogen is a well know process.As shown in FIG. 1, natural gas 90 and water 92 are reacted in a steamreformer 94 to form hydrogen 96 and carbon dioxide 98. Other methods toproduce hydrogen (partial oxidation of hydrocarbons, partial oxidationof coal, water electrolysis, etc.) could also be used. Where pure oxygenis available, such as in a fenceline operation with an air separationsplant, the partial oxidation processes can be economically viable. Whereinexpensive sources of electricity are available, electrolysis can beviable.

Another advantage of the current invention, compared to prior arttechnology for ethanol production, is the heat balance in the process.In the current invention, if hydrogen is made by steam reforming onsite, excess heat is available at high temperature and in an integratedplant due to the hydrogenation reaction of the ester being a highlyexothermic process. Therefore, the overall process is highly energyefficient. In addition, none of the carbohydrate raw material is wastedas CO₂ with the attendant generation of heat, which must be wasted tocooling water.

Another advantage of the current invention is the ability to convertnatural gas via hydrogen to a liquid product, e.g., ethanol, at veryhigh yield. This feature can be utilized in situations where anycarbohydrate source is located close to a source of natural gasproduction or easy transportation by pipeline. This allows theutilization of gas in remote geographies, such as islands that produceboth gas and sugar cane or other carbohydrate crop, to produce an easilytransported liquid chemical or fuel, again at high efficiency. Forexample, a plant using the process of the present invention could belocated on the island of Trinidad, where natural gas and carbohydratesources are available at economically attractive prices. The plant canproduce substantially pure ethanol for transport in liquid form to aremote location where it can be economically utilized, such as the TexasGulf Coast. The ethanol can be used as a fuel or a feedstock for furtherprocessing. For example, the ethanol can be converted to ethylene andsold through existing ethylene pipeline systems. Alternatively, theethanol can be recycled within the plant for production of ethylacetate. In other words, the plant can be used to produce a liquidproduct comprising substantially all ethyl acetate, substantially allethanol or any combination of the two. Because the natural gas andcarbohydrate source are located relatively close to each other, thesefeedstocks can be converted to a higher value liquid product which canbe easily transported to a remote location in an economic manner.

Preferably, the carbohydrate source and the natural gas source arelocated within five hundred miles of each other, more preferably withinthree hundred miles of each other and more preferably within two hundredmiles of each other. Preferably, the remote location to which thetransportable liquid product is located where economic transportation ofthe carbohydrate and natural gas is not viable, e.g., more than eighthundred miles, more preferably more than a thousand miles and morepreferably more than fifteen hundred miles from the point of production.It will be appreciated that it is generally easier to transport thecarbohydrate source to the source of the natural gas. It will also beappreciated that the specific distances that favor an economic advantagewill vary depending on the price of the feedstocks, the price of thetransportable liquid product and transportation costs. As will beappreciated by one skilled in the art, transportation costs areinfluenced by a number of factors, including geographic barriers, such amountain ranges, bodies of water, etc. and are not solely dependent ondistances. As a further example, natural gas and carbohydrate sourcescan be found in Australia and/or New Zealand. A plant located on theseisland nations could produce ethanol and/or ethyl acetate for transportto Asia, and in particular, Japan, Taiwan, Korea and China.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

1-75. (canceled)
 76. A method to produce ethanol, the method comprising:(a) culturing a homoacetogenic microorganism in a medium comprising acarbohydrate source to produce acetate, acetic acid or a mixturethereof; (b) forming a complex between an amine and the acetate; (c)forming acetic acid and regenerating the amine from the amine complex;(d) esterifying the acetic acid from the amine complex to an acetic acidester in the presence of an alcohol; and (e) hydrogenating the aceticacid ester to recover the ethanol.
 77. A method to produce ethanol, themethod comprising: (a) culturing a homoacetogenic microorganism,selected from the group consisting of a microorganism of the genusClostridium, a microorganism of the genus Acetobacterium, amicroorganism of the genus Peptostreptococcus, and mixtures thereof, ina medium comprising a carbohydrate source to produce acetate, aceticacid or a mixture thereof; (b) acidifying the acetate in the presence ofan amine to form an amine complex of the acetate; (c) recovering theamine complex of the acetate and decomposing it to form acetic acid; (d)esterifying the acetic acid to an ester, selected from the groupconsisting of methyl ester and ethyl ester; and (e) hydrogenating theester of acetic acid to form ethanol.
 78. A method to produce ethanol,the method comprising: (a) culturing a homoacetogenic microorganism in amedium comprising a carbohydrate source to produce acetate, acetic acidor a mixture thereof; (b) forming a complex between an amine and theacetate; (c) forming acetic acid and regenerating the amine from theamine complex; (d) esterifying the acetic acid from the amine complex toan acetic acid ester in the presence of an alcohol; and (e)hydrogenating the acetic acid ester to recover the ethanol; wherein atleast about 60% of carbon in the carbohydrate source is converted intothe ethanol.
 79. The method according to claims 76, 77 or 78, wherein atleast about 70% of carbon in the carbohydrate source is converted intothe ethanol.
 80. The method according to claims 76, 77 or 78, wherein atleast about 80% of carbon in the carbohydrate source is converted intothe ethanol.
 81. The method according to claims 76, 77 or 78, wherein atleast about 90% of carbon in the carbohydrate source is converted intothe ethanol.
 82. The method according to claims 76, 77 or 78, whereinnone of the carbon in the carbohydrate source is converted into carbondioxide.
 83. The method according to claims 76, 77 or 78, wherein thecarbohydrate source is selected from the group consisting of corn,wheat, biomass, wood, waste paper, manure, cheese whey, molasses, sugarbeets and sugar cane.
 84. The method according to claims 76, 77 or 78,wherein the homoacetogenic microorganism is a microorganism of the genusClostridium.
 85. The method according to claim 84, wherein themicroorganism of the genus Clostridium is a microorganism of the speciesClostridium thermoaceticum or Clostridium formicoaceticum.
 86. Themethod according to claims 76, 77 or 78, wherein the esterification isaccomplished by reactive distillation.
 87. The method according toclaims 76 or 78, wherein the homoacetogenic microorganism is selectedfrom the group consisting of a microorganism of the genus Clostridium, amicroorganism of the genus Acetobacterium, a microorganism of the genusPeptostreptococcus, and mixtures thereof.
 88. The method according toclaim 77, wherein the step of acidifying comprises reacting the acetatewith carbonic acid.
 89. The method according to claim 77, wherein theacidifying comprises reacting the acetate with carbon dioxide.
 90. Themethod according to claims 76, 77 or 78, wherein the carbohydrate sourceis enzymatically hydrolyzed to sugars and amino acids prior toculturing.
 91. The method according to claims 76, 77 or 78, wherein thepH of the medium is from about pH 6 to about pH
 8. 92. The methodaccording to claims 76 or 78, wherein the ester is selected from thegroup consisting of methyl ester, ethyl ester and mixtures thereof. 93.The method according to claims 76, 77 or 78, wherein the ester is ethylester.
 94. The method according to claims 76, 77 or 78, wherein theester is a volatile ester.
 95. The method according to claims 76 or 78,wherein the alcohol selected from the group consisting of methanol,ethanol, or mixtures thereof.
 96. The method according to claims 76, 77or 78, wherein hydrogen for the step of hydrogenation is produced by amember of the group consisting of a method comprising steam reforming, arenewable source, a biomass, and mixtures thereof.
 97. The methodaccording to claims 76, 77 or 78, wherein the medium comprises less thanabout 20% nitrogen.
 98. The method according to claims 76, 77 or 78,wherein the carbohydrate source is corn.